The present invention relates to reduction of harmonic and intermodulation distortion in an optical fiber network and more specifically to a circuit and method that pre-distorts the modulating input signal of a light emitting or laser diode to compensate the non-linear transfer characteristics of the diode.
In an optical fiber network, a large number of signals, e.g. TV signals, are frequency multiplexed to form a base band signal which is subsequently employed to modulate the output of a light emitting diode (LED) or laser diode. The intermodulation distortion between signals is caused by the non-linear transfer characteristics of the diode and may induce a significant level of interference in the network thereby distorting the output signals, reducing the efficiency of the network, or possibly rendering the network inoperable.
In U.S. Pat. No. 4,032,802, and apparatus is disclosed employing a compensating diode and a light emitting diode connected in parallel for DC biasing and in anti-series with respect to an AC signal source. The patent asserts that the non-linear junction capacitance of the LED is responsible for the undesirable harmonic distortion and intermodulation products. To reduce distortion problems, a compensating diode matching the LED as nearly as possible with respect to capacitance characteristics is selected, with the bias point of the diode being adjusted to equalize the effects of the diode capacitance. This method may reduce harmonic and intermodulation distortion in the LED to some extent but will not totally eliminate the distortion products in the output signal. Additionally, the LED will saturate as the input current is increased, and the diode's output power will fall off accordingly. The output power of the diode plotted with respect to the input current falls off non-linearly as the current is increased, as shown by curve 20 in FIG. 1.
FIG. 2 is a simplified view of a fiber optic communication network 22 wherein an input signal 60 modulates an optical signal (not shown) of an LED 56 to form a modulated output signal 68 which is transmitted through an analog optic fiber network 70 where the modulated signal 68 is detected by a detector 32 thereby providing an output signal 69 corresponding to input signal 60. It is highly desirable that the detected output signal 69 vary linearly with respect to the input signal 60 modulating the LED. If, however, the transfer curve of the LED exhibits the non-linear characteristics previously described, the output signal will also vary non-linearly with respect to the input signal indicating the existence of intermodulation products between channels and distortion in the network.